Steels conventionally used in making such sheets generally have the following composition, by weight:
______________________________________ carbon 0.05 to 0.07% manganese 1.5 to 2.0% titanium 0.01 to 0.04% sulfur 0.001 to 0.003% nitrogen 0.005 to 0.008% silicon 0.25 to 0.40% aluminum 0.03 to 0.05% niobium up to 0.08% remainder iron and usual impurities (possibly including calcium) ______________________________________
Slabs formed by continuous casting from such a composition, at a temperature of not more than 850.degree. C., are thermomechanically treated with a degree of deformation of at least 60% in an initial hot-rolling stage, followed by final hot rolling at a temperature of 750.degree. to 650.degree. C.
Reference in this connection may be made, for example, to German laid-open specifications Nos. 30 12 139 and 31 46 950. According to the processes described there, the proportion of titanium lies in the range of 0.008 to 0.025% by weight, without observation of any particular ratio between the amounts of titanium and nitrogen. As is apparent from the foregoing table, the art does not consider niobium an essential alloying ingredient. The requisite hardening and grain refining are conventionally controlled by the precipitation of titanium nitride, TiN, which the art endeavors to create in the form of a multitude of fine particles not exceeding 0.05 micron in size. The conventional technique therefore involves a rapid cooling of the slabs after their continuous casting, care being taken to prevent the coarsening of the TiN precipitates during the further treatment so that the fine particles are preserved after final rolling. For the latter purpose it has been proposed to limit the annealing temperature of the slabs before rolling to a range of 950.degree. to 1050.degree. C. (German specification No. 31 46 950) or 900.degree. to 1000.degree. C. (German specification No. 30 12 139). The fine TiN precipitates are expected to impede the growth of austenitic grains and to obviate the formation of coarse particles in the thermally affected zones of weld seams.
Under certain conditions of use, as where pipes are to carry fluids at substantial pressure in regions of permanent frost, the mechanical properties of conventionally produced steel sheets fail to satisfy essential criteria of tensile strength and elastic limit, for example. Attempts have been made to improve these properties by the addition of niobium, usually in combination with substantial quantities of such relatively costly metals as vanadium, nickel and chromium. At least in the absence of these latter metals, however, the niobium does not significantly contribute to the stress resistance of steel sheets whose hardness is predominantly determined by TiN precipitates. The reasons for the unsatisfactory performance of niobium, I have found, reside in the insufficient solution thereof at the relatively low annealing temperature of the continuously cast slabs as well as in the formation of counterproductive compounds. Thus, a low proportion of titanium favors the formation of strength-reducing niobium carbon nitride, NbCN, whereas an excessive percentage of titanium leads to the formation of ductility-impairing titanium carbide, TiC.